1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices, and more particularly, to improving the image quality of LCD devices.
2. Discussion of the Related Art
Because of their advantages of low power consumption, lightweight, and small size, liquid crystal displays (LCDs) are highly attractive alternatives to cathode ray tubes (CRTs) in many applications.
An LCD device typically includes arrays of switching devices and pixel electrodes on an array substrate, a filter substrate having a common electrode, a color filter array and a black matrix, and a liquid crystal layer that is disposed between the array and filter substrates. The switching devices control the applications of signal voltages to the pixel electrodes. The pixel electrodes interact with the liquid crystal layer to selectively transmit light from pixel regions in response to the applied signal voltages. The array substrate further includes an array of storage capacitors for maintaining pixel information between applications of the signal voltages. The common electrode acts with the pixel electrodes to produce electric fields across the liquid crystal layer. The color filter layer colors the selectively transmitted light. The black matrix prevents light from entering regions where liquid crystal arrangement is not controllable.
Each storage capacitor maintains the signal voltage applied to a pixel electrode when the switching transistor (typically a thin film transistor) for that pixel electrode is turned-off. The purpose of the storage capacitor is to prevent picture quality deterioration. Storage capacitors are of a storage capacitance type or a supplemental capacitance type, with the distinction depending on how the capacitor electrodes are formed.
In the storage capacitance type, a distinct storage electrode is provided for each storage capacitor. In the supplement capacitance type, part of a gate line acts as a storage electrode. In general, LCD devices that use supplement capacitance type storage capacitors can have higher aperture ratios (because a separate storage electrode is not required) and typically have a higher fabrication yield (because of simplicity). However, LCD devices that use supplement capacitance type storage capacitors generally have relatively poor picture quality, at least partially because supplement capacitance type storage capacitors are not well suited to dot-inversion and column inversion drive systems.
On the other hand, LCD devices that use storage capacitor type storage capacitors often have lower aperture ratios, but better picture quality. Accordingly, LCD devices that use the storage capacitance type storage capacitors are better suited for video displays in which the low aperture ratio is acceptable.
A related art LCD device that uses supplement capacitance type storage capacitors will be described with reference to FIG. 1 and to FIG. 2. FIG. 1 is a simplified schematic view of the related art LCD device, while FIG. 2 provides an equivalent circuit diagram of a pixel. That LCD device includes gate lines 11 that are formed on a first substrate, an insulating film (not shown) formed over the entire surface of the first substrate, including the gate lines 11, and data lines 14 that cross the gate lines 11 to define pixel regions. Upper capacitor electrode 14c, beneficially formed at the same time as the data lines, are located over the gate insulating film and over predetermined portions of the gate lines 11. Switching devices are located near crossings of the gate lines 11 and the data lines 14. A passivation film (not shown) having a predetermined thickness is beneficially formed over the entire surface of the first substrate, including over the switching devices. Pixel electrodes 19 of indium tin oxide (ITO) are connected to the switching devices and to the upper capacitor electrodes 14c through first and second contact holes 17 and 18.
Still referring to FIG. 1, predetermined portions of the gate lines 11 serve as lower capacitor electrodes. Accordingly, each storage capacitor includes an upper capacitor electrode 14c, part of a gate line 11, and the gate insulating film, which is interposed between the upper capacitor electrode 14c and the gate lines 11.
Referring now to FIG. 2, a parasitic capacitance Cgs results from the gate electrode G crossing the source/drain electrodes S/D. This parasitic capacitance induces a direct current (DC) voltage offset, Vp, when an AC voltage is applied to the liquid crystal. The DC voltage offset Vp results in undesirable effects, such as flicker, image “sticking,” and uneven screen brightness. To solve such problems, the storage capacitor provides a storage capacitance Cst that improves picture quality by reducing changes in Vp. The storage capacitance Cst increases as the size of the storage capacitor electrode increases. However, if the storage capacitor electrode increases too much, the aperture ratio is reduced. Therefore, an optimal storage capacitor electrode size should be maintained.
Unfortunately, the overlap of the gate electrode 11a and the source/drain electrodes 14a and 14b may be vary because of photolithography errors. In that case, the parasitic capacitance can increase, and thus the desired Vp value cannot be obtained.
For reference, the remaining elements of FIG. 2 will be described. D.L denotes data lines 14, to which bipolar signal voltages are applied, while G.L denotes gate lines 11, to which scanning signals are applied. Clc denotes charge capacitance stored in the interval between a pixel electrode and the common electrode Vcom, while Cst denotes a charge capacitance stored in the interval between the predetermined part of the gate lines 11 and the upper capacitor electrode 14c. 
Referring once again to FIG. 1, each switching device includes a gate electrode 11 a that is diverged from a gate line 11, a gate insulating film (not shown, but located over the entire surface of the first substrate, including the gate lines 11), a semiconductor layer 13 on the gate insulating film and over the gate electrode 11a, and source/drain electrodes 14a and 14b at ends of the semiconductor layer 13. The switching device beneficially is an amorphous silicon thin film transistor (a-Si TFT).
To complete the LCD device, the array substrate, provided with the above elements, is attached to a color filter substrate having a black matrix, a red, green, and blue (R, G, B) color filter layer, and an ITO-based common electrode. A liquid crystal is disposed between the two attached substrates.
While generally successful, the related art LCD device has problems. Even though an effort is made to reduce changes in Vp by restraining the parasitic capacitance, Vp is not well controlled because of parasitic capacitance deviations. Such deviations can be caused by misalignment of the gate electrodes and the source/drain electrodes, which can occur because of process errors in critical dimensions (CD) and photolithography.
Large-screen display devices are particularly susceptible to capacitance problems because such displays are highly sensitive to flicker and/or picture brightness problems. Therefore, an LCD device that is compensated for parasitic capacitance deviations would be beneficial. Even more beneficial would be an LCD device that maintains a more uniform Vp.